1. Field of the Invention
The present invention relates to an optical scanning apparatus.
2. Description of the Related Art
In an image forming apparatus adopting electrophotography, a light beam of a laser has previously been allowed to deflection-scan (e.g., by using a polygon mirror rotating at a high speed), and been applied to a photosensitive body. That is, a latent image is formed on a photosensitive body, the surface of which is uniformly electrified with a charger, by light irradiation in accordance with the image information. Subsequently, the resulting latent image is developed with a developer, and the developed image is transferred to a recording material, so that an image is formed.
The polygon mirror in the above-described image forming apparatus is coaxially attached to a rotating shaft of a brushless motor to constitute a polygon motor, and is disposed in a light beam scanning apparatus of the image forming apparatus, so as to be driven to rotate at a high speed of 20,000 rpm to 50,000 rpm.
However, when this polygon motor is rotated at a high speed, airflow is generated, and a noise problem occurs. In order to overcome this problem, previously, the polygon motor has been hermetically sealed and leakage of the sound to the outside of the apparatus has been reduced. For example, as described in Japanese Patent Laid-Open No. 2000-330055, a polygon motor is disposed in a hermetically sealed space partitioned by a lid, and case.
Even when the space is hermetically sealed, wind noise of the polygon mirror occurs. A configuration to reduce this is described in Japanese Patent No. 3472142. As is indicated by a schematic diagram shown in FIGS. 10A-B, the polygon motor 11-7 is hermetically sealed, and an annular slit 40 open in the direction all around the perimeter of the polygon mirror 11-10 and extending in an outward direction of the polygon mirror is disposed in a cover component provided with an cylindrical portion 104 covering the outer perimeter of the polygon mirror. In this manner, the airflow in the rotating surface direction in the cover is made uniform in the circumferential direction, and the noise is reduced effectively.
However, according to the above-described document, even when the cover is disposed, the air pressure cannot be made uniform due to an airflow generated in the rotation axis direction of the polygon motor (hereafter referred to as a vertical direction).
Since the polygon mirror is rotated, an airflow, which flows outward in a polygon mirror radius direction, is generated partly on a polygon mirror upper surface portion due to the viscosity of air, and the polygon mirror upper surface becomes at a negative pressure. Likewise, for a motor rotor, an airflow is generated in the vertical direction depending on the shape and, thereby, a pressure gradient is generated. As a whole, these flows are synthesized, and a large airflow is generated in the rotation axis direction of the polygon motor. In particular, if the polygon mirror upper surface portion or lower surface portion develops a negative relative pressure (with respect to the middle rotation surface) and a pressure gradient is generated, variations in the pressure applied to the reflection surface of rotating noncircular polygon mirror becomes larger in the rotation direction, and a problem may occur in that the rotational variation of the polygon mirror under high accuracy rotation control is deteriorated. If the rotational variation of the polygon mirror occurs, the length of scanning on the photosensitive drum is varied, and a poor image may result, in which distortion occurs particularly in an image on the scanning end side, or a poor image, e.g., color misalignment, may result in a full color printer which produce an image by superimposing images of a plurality of colors.
In recent years, the diameter of a polygon mirror has been decreased by adopting a semiconductor laser having multiple emission points as a light source and decreasing the number of reflection surfaces. Consequently, the power required for driving the polygon mirror to rotate can be reduced on the basis of reduction of the rotational inertia, the start-up time can be reduced, and light beam scanning apparatus can be miniaturized. On the other hand, since the rotational inertia is reduced by the decrease of the diameter of the polygon mirror, the above-described rotational variation tends to become particularly significant.
FIG. 11 is a sectional view of a polygon motor disposed in a hermetically sealed space partitioned by a lid component and a case, as described in Japanese Patent No. 3472142. Arrows (F11, F12) shown around the polygon motor indicate the result of flow analysis of the airflows generated during the rotation of the polygon motor 11-2. Particularly, in a configuration in which a circumscribed circle diameter of the polygon mirror 11-1 is smaller than the motor rotor diameter, as shown in FIG. 11, the air flows downward from above the entire polygon mirror, as indicated by arrows F12. In particular, above the polygon mirror upper surface portion, spiral flows as indicated by arrows F11 are generated and the inflow of air is restricted, so that a negative pressure portion 150 is generated, the flow indicated by arrows F12 exerts a synergistic effect, and a significant pressure gradient is generated in the vertical direction. Consequently, the pressure gradient in the vertical direction becomes further significant, and the negative pressure generated at the polygon mirror upper surface may exceed 100 Pa.
Therefore, it is desired that the pressure gradient in the vertical direction is minimized in the rotation axis direction of the rotating polygon mirror.